Chemotaxis is the ability of living systems to sense chemical gradients in their surroundings and react accordingly. Most prominent chemotactic behavior is exhibited by simple microorganisms that are able to migrate towards gradients of concentrations of the chemoattractant, via activation of the complex intracellular sensing cascades making use of specific chemoreceptors. Classical demonstrations of this phenomenon were described by Engelmann, who observed the accumulation of Bacterium termo, a rod-shaped bacterium, in oxygen-rich surroundings of cells undergoing photosynthesis. Yet, other bacteria can show completely different behavior, like Spirillumtenue, which is repelled by high oxygen pressure, presenting an antichemotactic behavior.